The present invention generally relates to a radio receiver for receiving an FMX stereophonic broadcasting and more particularly, to an FMX stereophonic broadcast receiver provided with countermeasures against transient noises.
FMX stereophonic broadcasting has been proposed as one means for expanding the service area and improving the signal-to-noise ratio characteristics of FM stereophonic broadcasts. The transmission signal of the aforementioned FMX stereophonic broadcast includes a compressed stereo difference signal (L+R)' broadcast simultaneously with the transmission signal of conventional FM stereophonic broadcasting, for example, a stereo sum signal (L+R) and stereo difference signal (L-R). The transmission signal can be expressed as: EQU f(t)=(L+R)+Psin(.omega./2)t+(L-R)sin.omega.t+(L-R)'cos.omega.t(1)
where L+R is a stereo sum signal, L-R is a stereo difference signal P, is a stereo pilot signal, and .omega. is the subcarrier angular frequency. As shown by the aforementioned Equation (1), compressed stereo difference signal (L-R)' is quadrature modulated from uncompressed stereo difference signal (L-R), resulting in an FMX stereophonic broadcast transmission signal spectrum shown in FIG. 4.
Furthermore, the relationship between the uncompressed stereo difference signal (L-R) and the compressed stereo difference signal (L-R)' is as shown in FIG. 5 which expresses the compression characteristics. In FIG. 5, when the input signal level is low, the aforementioned signal (L-R)' is 20 dB greater than the uncompressed stereo difference signal (L-R) and, at the same time, input/output characteristics become linear, and also the compression ratio becomes 1 :1. When the level of the input signal is medium (approximately -30 dB), the compression ratio becomes .infin.:1, and input/output characteristics are flat over a range of approximately 10 dB. When the input signal level becomes high, the aforementioned signal (L-R)' rapidly attenuates. Therefore, compressed stereo difference signal (L-R)' is as shown by solid line (b) in FIG. 5 with respect to stereo difference signal (L-R) (solid line (a)), and the sum signal of the aforementioned signal (L-R) and the aforementioned signal (L-R)' is as shown by dotted line (c) in FIG. 5.
The transmission signal for FMX stereophonic broadcasting as described above is received by a conventional receiver as shown in FIG. 7. In FIG. 7, the FMX stereophonic broadcast transmission signal received by antenna 1 is received by a receiving circuit 2 of the same construction as a conventional FM stereophonic receiver in which stereo sum signal (L+R) (hereinafter referred to as M), stereo difference signal (L-R) (hereinafter referred to as S), and compressed stereo difference signal (L-R)' (hereinafter referred to as S') are each demodulated. When the received signal is detected by the FM detection circuit (not shown) included in the receiving circuit 2, the stereo sum signal M is demodulated. When the stereo composite signal is subjected to synchronous detection through employment of the 38-KHz subcarrier signal obtained from a PLL in the receiving circuit 2, uncompressed stereo difference signal S is demodulated. And when the stereo composite signal is subjected to quadrature detection, the compressed stereo difference signal S' is demodulated.
The uncompressed and compressed stereo difference signals S and S' obtained from the receiving circuit 2 are added by an adder 3, and the result is supplied to a VCA (voltage control amplifier) 4 operating as an attenuator. When the stereo difference signal S and output signal (S+S') of the VCA 4 are greater than a specified level (a knee-point level), first and second level detection circuits 5 and 6, each having a threshold level, operate in such a manner that the level of stereo difference signal S and the level of the aforementioned output signal (S+S') of the VCA 4 are respectively detected by the first and second level detection circuits 5 and 6, and are compared by a comparison circuit 7. Next, a signal according to the level difference obtained from the aforementioned comparison circuit 7 is rectified and smoothed by a rectifying circuit 8, and the rectified signal is applied to the VCA 4 as a control signal. The output signal (S+S') of the aforementioned VCA 4 is controlled by this control signal to be equal to the level of the stereo difference signal S. However, when the aforementioned stereo difference signal S and output signal (S+S') of the VCA 4 are below the knee-point level, the first and second level detection circuits 5 and 6 do not operate, and attenuation at the VCA 4 is fixed at approximately 20 dB.
Although the stereo sum signal M obtained from the receiving circuit 2 is applied directly to a matrix circuit 9, and the stereo difference signal S and output signal (S+S') of the VCA 4 are selected by a switch 10, and applied to the matrix circuit 9. A 10Hz ID signal is included in the FMX stereophonic broadcast transmission signal, and FMX stereophonic broadcasts are differentiated from conventional FM stereophonic broadcasts by the aforementioned ID signal. In addition, because an ID detection circuit (not shown) which detects the aforementioned ID signal is incorporated in the receiving circuit 2, whether the broadcast is FMX stereo or not can be determined by the output signal of the ID detection circuit. The switch 10 is controlled by the aforementioned ID signal. When the ID signal (such as a HIGH level signal) is present, the switch 10 is switched to a position as shown in FIG. 7. Accordingly, the stereo sum signal M and output signal (S+S') from the level controlled VCA 4 are matrixed, and left and right stereo signals L and R are generated at left and right output terminals 11 and 12. Furthermore, when the ID signal is not present, the switch 10 is switched to a position opposite to that shown in FIG. 7, and stereo sum signal M and stereo difference signal S are matrixed in the matrix circuit 9. As described above, because FMX stereophonic broadcast system uses compressed and expanded stereo difference signal S, it is possible to achieve significant improvements in the S/N ratio, and the service area can be expanded generally equal to that of the conventional monaural FM broadcast system.
It is to be noted that the FMX stereophonic broadcast transmission signal can be accurately received by a conventional FM stereophonic receiver. In this case, the compressed stereo difference signal S' is quadrature modulated with respect to stereo difference signal S, and reception is not adversely affected.
Details concerning FMX stereophonic broadcasting are disclosed, for example, in an article "Improving the Signal-to-Noise Ratio and Coverage of FM Stereophonic Broadcasts" by Emil L. Torick and Thomas B. Keller in "JOURNAL OF THE RADIO ENGINEERING SOCIETY", volume 33, number 12, issued December 1985.
Incidentally, in the reception of the FMX stereophonic broadcasting, the noise level is raised by approximately 20 dB at the maximum in the degree of modulation above the knee point (point A) as shown by a solid line (a) in FIG. 6, and there has been a problem that such rising of the noise level tends to be offensive to the ear, thus spoiling pleasant feeling in listening during reception of a signal having a level low on the average but increasing temporarily, e.g. playing sound of a piano or the like.
Meanwhile, it is known that the noise level of a receiving signal increases as the field strength of said receiving signal becomes weak. Therefore, influence of the noise level becomes remarkably larger as the field strength becomes weaker, with respect to the variation of the noise level corresponding to the degree of modulation referred to earlier.